Field
The present disclosure relates generally to switching amplifiers, and, more specifically, to high-power, high-efficiency, broadband DC to RF switching amplifiers.
Background
RF amplifiers with active devices acting as current sources, such as the well-known Class A, B, and C amplifiers, cannot achieve 100 percent efficiency. To achieve a theoretical efficiency limit of 100 percent, the active devices need to act as switches that are either on or off, thus eliminating resistive losses in the switches. Examples of such amplifiers are the well-known Class D, E, and F amplifiers.
Class D, E, and F switched amplifiers generate voltages or currents over or through the device that are not sinusoidal and, thus, require filters to generate a spectrally pure sinewave output. The need for the output filter makes it difficult to design an amplifier that can generate power from a very low frequency (including possibly direct current (DC) operation) to some maximum radio frequency (RF). For example, to operate at a frequency of 1 MHz generally requires an output filter that suppresses the third and higher harmonics. That is, the output filter needs to allow 1 MHz to pass but reject frequencies from 3 MHz to, say, 100 MHz. This means that the same amplifier will not be able to operate efficiently at 10 MHz because 10 MHz is being rejected by the output filter. This problem can be overcome by keeping the switching frequency high and modulating the switch waveform to generate the desired output frequency. This gives rise to the Class S amplifier.
One limitation of the Class S amplifier is that switching losses can be high in actual implementations. A significant portion of switching losses is due to the output capacitance that is inherent in all known semiconductor switches. When the device switches from the off state to the on state with a voltage over the device, the energy stored in the output capacitance of the device is dissipated in the switch. The Class E amplifier includes an output filter designed such that, in the ideal case, the device never switches on when there is a voltage over the device. With Class D amplifiers, controlling the dead time and inductive pre-loading can achieve zero-voltage switching. However, in the case of the Class S amplifier, the switching frequency is generally higher than the frequency of the output, so the switching occurs at a variety of voltages over the device, and it is generally not possible to avoid these losses by clever design of the output filter. Most semiconductor devices have output capacitances that decrease approximately as the square root of the applied voltage so that the stored energy is approximately proportional to the applied voltage. In addition to capacitive switching losses, there are also losses associated with the time it takes to turn the device fully on or off. During the transition time, both voltage over and current through the switch are present at the same time, leading to losses. These losses are also approximately proportional to the maximum voltage over the device during the transition.
It is thus apparent that there is a need in the art for an improved broadband switching amplifier.